Apparatus for reducing porosities in composite resin parts

ABSTRACT

An electrical charge is placed on a tool used to cure a composite resin part layup. The charged tool produces an electrostatic force that attracts entrapped gases in the resin to the surface of the tool, thereby reducing porosities in the cured part.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a division of and claims the benefit of priority toU.S. patent application Ser. No. 13/488,768, filed on Jun. 5, 2012, thecontents of which are incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under contract numberW31P4Q-09-D-0029, awarded by the United States Army. The Government hascertain rights in this invention.

BACKGROUND INFORMATION

1. Field

The present disclosure generally relates to the fabrication of compositeresin parts, and deals more particularly with a method and apparatus forreducing porosities in the parts.

2. Background

Composite resin parts may be fabricated by laying up multiple plies offiber reinforced resin in the form of prepreg. The prepreg part layup iscured by placing it on a tool and subjecting it to a combination of heatand pressure. As the part layup is heated, the prepreg plies soften andflow to form a consolidated structure, however air and/or volatile gasesmay be entrapped within the plies during the curing process that resultin porosities in the cured part. These porosities are undesirablebecause they may reduce the performance of the part. Entrapped airand/or gases near the tool side of the part layup are particularlyproblematic and difficult to remove in some applications. One solutionto the problem of tool-side part porosity involves curing the part layupin an autoclave where high pressures are applied to the part that mayforce out entrapped air and/or gases. Autoclave curing of composite partlayups is time consuming, labor intensive and requires large, relativelyexpensive capital equipment. Another solution to the problem focuses onallowing the part layup to breathe through outer skins of the layupusing bagging techniques that employ embedded breathing materials suchas glass or polyscrim materials. The extensive use of breathingmaterials, including embedded breathing materials when used in ovencuring is also time consuming and labor intensive, and may not becompletely effective in eliminating part porosities, particularly thoseoccurring near the tool-side of the part.

Accordingly, there is a need for a method and apparatus for curingcomposite resin part layups that substantially reduces or eliminatestool-side part porosities. Further, there is a need for a method andapparatus as mentioned above which allows out-of-autoclave processing ofa composite resin part layup using conventional vacuum baggingtechniques and curing within a conventional oven.

SUMMARY

The disclosed embodiments provide a method and apparatus forout-of-autoclave curing of composite resin part layups that may becarried out using conventional ovens, and which is effective in reducingor eliminating tool-side part porosities. Tool-side part porosities arereduced by placing a charge on a cure tool using an electric chargegenerator, such as, without limitation, an electrostatic chargegenerator. The reduction of tool-side porosities may enable parts to becured using out-of autoclave processes.

In one embodiment, the electrical charge generator is used to negativelycharge the tool, causing a negative charge to be placed on the toolsurface engaging the part layup. The part layup carries a positivecharge. The negative charge on the tool surface forms a charge imbalanceor potential difference between the cure tool and the positively chargedpart layup, and this potential difference results in an electrostaticattractive force. As the part layup is heated during the cure process,the resin becomes less viscous and begins to flow, allowing anyentrapped gas molecules to migrate. The electrostatic force attractsmolecules of both the entrapped gasses and the resin toward the toolsurface, thereby substantially reducing or eliminating part porosities,especially tool-side porosities.

The negative charge may be placed on the tool using a mechanicaltechnique such as without limitation, triboelectric charging. Thetriboelectric charging is achieved by placing two materials on thebackside of the tool that are sufficiently far apart on thetriboelectric series to produce the necessary potential differencebetween the uncured composite part layup and the tool surface. In otherembodiments, the negative charge may be placed on the tool using adynamically powered system, such as a Van der Graaff generator.

According to one disclosed embodiment, a method is provided of reducingporosities in a composite resin part. The method comprises placing anuncured composite part layup on a surface of a tool and placing anelectrical charge on the tool by electrically charging the tool. Themethod further comprises using the electrical charge on the tool toattract molecules within the uncured composite part layup to the surfaceof the tool, and curing the composite part layup. The electricalcharging of the tool may be performed using triboelectric charging or aVan der Graaff generator. Charging is performed by placing a negativecharge on the tool, which is used to attract molecules of gasses in thecomposite part layup toward the tool surface.

According to another embodiment, a method is provided of fabricating acomposite part, comprising placing an uncured composite resin part layupon a surface of a tool and sealing a vacuum bag over the composite partlayup. The method further comprises drawing a vacuum in the bag, heatingthe composite part layup, and placing an electric charge on the tool toattract gas molecules within the composite resin part layup towards thesurface of the tool. The method also includes curing the composite partlayup. The curing process may be carried out within an oven. Placing theelectric charge in the tool may be performed by triboelectrical chargingof the tool.

According to still another embodiment, a method is provided of reducingtool-side porosities in a composite resin part layup while being curedon the surface of the tool. The method comprises using an electriccharge to attract gas molecules in a composite resin part layup to thetool surface.

According to still another embodiment, apparatus is provided for curinga composite resin part, comprising a tool having a tool surface adaptedto engage a composite resin part layup, and an electrical chargegenerator for generating a charge on the tool surface sufficientmagnitude to attract gas molecules in the composite resin part layup tothe tool surface.

The features, functions, and advantages can be achieved independently invarious embodiments of the present disclosure or may be combined in yetother embodiments in which further details can be seen with reference tothe following description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features believed characteristic of the advantageousembodiments are set forth in the appended claims. The advantageousembodiments, however, as well as a preferred mode of use, furtherobjectives and advantages thereof, will best be understood by referenceto the following detailed description of an advantageous embodiment ofthe present disclosure when read in conjunction with the accompanyingdrawings, wherein:

FIG. 1 is an illustration of a functional block diagram of apparatus forcuring a composite resin part layup that reduces part porosities.

FIG. 2 is an illustration of an exploded side view of one embodiment ofthe apparatus shown in FIG. 1.

FIG. 3 is an illustration of an unexploded sectional view taken alongthe line 3-3 in FIG. 2.

FIG. 4 is an illustration of a side view of another embodiment of theapparatus

FIG. 4A is an illustration a diagrammatic view of a Van der Graaffgenerator employing the triboelectric effect.

FIG. 5 is an illustration of a side view of a further embodiment of theapparatus.

FIG. 6 is an illustration of a flow diagram of a method of curing acomposite resin part layup that reduces part porosities.

FIG. 7 is an illustration of a flow diagram of aircraft production andservice methodology.

FIG. 8 is an illustration of a block diagram of an aircraft.

DETAILED DESCRIPTION

FIG. 1 broadly illustrates the components of apparatus for curing acomposite resin part layup that reduces part porosities due to entrappedgases. The reduction in part porosities, particularly tool-sideporosities, may enable the use of out-of-autoclave processes to curecomposite part layups. An uncured composite part layup 20 is placed onthe surface 25 of a suitable tool 22, sometimes referred to as a curetool or a bond tool. As will be discussed in more detail below, a vacuumbag (not shown in FIG. 1) may be placed over the part layup 20 andsealed to the tool surface 25 in order to apply compaction pressure tothe part layup 20 during the cure process. The part layup 20 placed onthe tool 22 may be cured in a conventional oven indicated by the brokenline 26. In order to reduce part porosities, particularly partporosities near the tool surface 25, an electric charge generator 24 isused to place a negative charge − on the tool 22 which may be maintainedthroughout the cure cycle. The uncured composite part layup 20 carries apositive charge +. The potential difference V resulting from thepositively charged part layup 20 and the negatively charged tool 22produces an electrostatic force F that attracts entrapped air and/orvolatile gas molecules 58 (hereinafter collectively referred to as gas,gasses or gas molecules) to the surface 25 of the tool 22. As a resultof the migration of the gas molecules 58 to the tool surface 25,porosities in the cured part layup 20 caused by entrapped gasses aresubstantially reduced or eliminated.

Attention is now directed to FIGS. 2 and 3 which illustrate oneembodiment of the apparatus shown in FIG. 1 that may be employed to curea composite resin part layup 20 within conventional oven 26 usingconventional vacuum bagging techniques to apply compaction pressure tothe part layup 20 during the cure process. The uncured composite partlayup 20 may comprise multiple plies of a prepreg, each including afiber reinforcement 20 a (FIG. 3) held in a polymer resin matrix 20 b.The uncured composite part layup 20 is placed on surface 25 of a tool22. In the illustrated embodiment, the tool surface 25 is shown as beingsubstantially flat, however in other applications the tool surface 25may have one or more contours or curves or a combination of flat areasand contours (not shown). The tool 22 may comprise metal, a composite orother materials capable of maintaining an electric charge that isproduced by an electric charge generator 24, which in this example is anunpowered, mechanical form of the electric charge generator 24 shown inFIG. 1.

A conventional flexible vacuum bag 32, which may comprise withoutlimitation, nylon or polyester, covers the part layup 20 and is sealedto the tool surface 25 using conventional sealing tape or other sealingmethods (not shown). Although not illustrated in the drawings, one ormore breathers, peel plies, caul plates, etc. may be placed over thepart layup 20 beneath the vacuum bag 32. The vacuum bag 32 includes anair outlet 38 coupled with vacuum tube 40 that is used to evacuate thebag 32, resulting in compaction pressure being applied to the part layup20 during a cure cycle.

An electric charge generator 24, described later, is attached to thebottom side 36 of the tool 22. The electric charge generator 24 alongwith the tool 22 and the part layup 20 are supported on an oven rack 30that may be placed within a conventional heating oven 26 to carry outcuring. The electric charge generator 24 is electrically insulated fromthe oven rack 30 by an insulation layer 28, which may comprise, withoutlimitation, a glass fabric. During charging of the tool 22 by theelectric charge generator 24, the insulation layer 28 electricallyinsulates the electric charge generator 24 from the oven rack 30, thuspreventing discharge of the negative charges placed on the tool 22.

In the embodiment shown in FIGS. 2 and 3, the electric charge generator24 comprises a triboelectric static charge generator that includes firstand second layers 42, 44 respectively, of a first material, spaced apartand separated by pervious layer 46 of a second material that allows airto pass therethrough. The first and second layers 42, 44 respectively ofthe first material, have opposing surfaces 42 a, 44 a (FIG. 3) that arein contact with the layer 46 of the second material. The triboelectriccharge generator 24 generates an electric charge by the triboelectriceffect. The triboelectric effect, sometimes known as triboelectriccharging, is a form of contact electrification in which certainmaterials become electrically charged after they come in contact withanother different material, and are then separated, such as throughrubbing, although charge transfer may occur in some cases through simplecontact without rubbing or separation. The polarity and the strength ofthe charges produced differ according to the materials, surfaceroughness, temperature, strain and other properties of the materials.Materials may be arranged in a list, known as the triboelectric series,according to the order of a polarity of charge separation when they aretouched by another object. A material towards the bottom of the series,when touched by a material near the top of the series, will attain amore negative charge, and vice versa. The further away materials arefrom each other on the triboelectric series, the greater the chargetransferred.

The layers of material 42, 44, 46 are covered by a vacuum bag 56 whichmay comprise for example, and without limitation, polyester or nylon.The vacuum bag 56 is sealed to the bottom side 36 of the tool 22 by anysuitable technique, such as use of a conventional sealing tape (notshown) and conformally engages one the second layer 44 of the secondmaterial. A Venturi device 52 attached to one side of the bag 56 iscoupled with a suction tube 54. The Venturi device 52 includes aninternal Venture tube (not shown) that produces a local partial vacuumwithin the bag 56. This local partial vacuum causes air to be drawn intoan inlet 48 in the bag 56 and through the layer 46 of the secondmaterial.

The air flowing through the layer 46 of the second material moves acrossover the surfaces 42 a, 44 a (FIG. 3) of the first and second layers 42,44 of the first material. The second material forming layer 46 and thefirst material forming the layers 42, 44, are chosen such that they aresufficiently spaced apart in the triboelectric series to generate thedesired magnitude of negative charge on the tool 22 when air flowsthrough the layer 46 and over the first and second layers 42, 44. Forexample, and without limitation, in one embodiment, each of the firstand second layers 42, 44 of material may comprise a material that has arelatively negative charge in the triboelectric series, such as, withoutlimitation, a suitable FEP (fluorinated ethylene propylene) such asTeflon®, and the layer 46 of the second material may comprise a wovenglass/N10 having a relatively positive charge in the triboelectricseries.

Contact of the bag 56 with second layer 44, along with air flow 50 overthe opposing surfaces 42 a, 44 a of the first and second layers 42, 46respectively result in a negative charge being placed on the tool 22 bya triboelectric charge generation effect. Other arrangements ofmaterials may be employed on the backside 36 of the tool 22 in order togenerate an electrostatic charge on tool 22 through a triboelectriceffect. As previously discussed, the electric charge produced by theelectric charge generator 24 negatively charges the tool 22. Thenegative charge on the tool surface 25 results in an electrostatic forceF (FIG. 1) that attracts molecules (FIG. 3) of both the gases and theresin 58, 60 respectively, in direction 62 toward the tool surface 25,thereby substantially reducing or eliminating porosities in the curedpart, particularly tool-side porosities. The migration of the resinmolecules 60 toward the tool surface 25 caused by the electrostaticforce F may aid in moving the gas molecules 58 out of the part layup 20to the tool surface 25.

As previously mentioned, the electric charge generator 24 shown in FIG.1 may comprise any of a number of devices that are capable of generatingan electric charge that may be transferred to the tool 22 in order toestablish a negative electrostatic charge on the tool surface 25 and thedesired potential difference between the tool 22 and the composite partlayup 20. For example, referring to FIG. 4, the electric chargegenerator 24 may comprise a powered electrostatic charge generator 64that may be directly connected to the tool 22. In this example, the tool22 is supported on an oven rack 30 that is insulated from the tool 22 bya layer 28 of electrical insulation.

Electrostatic generator 64 may comprise, without limitation, a Van derGraaff generator comprising a belt (not shown) of flexible dielectricmaterial running over two or more pulleys (not shown), and electrodespositioned near the pulleys (not shown). Alternatively, theelectrostatic generator 64 may comprise a form of a Van der Graaffgenerator employing the triboelectric effect, such as that shown in FIG.4A. In this latter example, the friction between one or more belts 65and a pair of rollers 67, 69, one of them being made of insulatingmaterial, or both being made of insulating materials at differentpositions on the triboelectric scale, one above and the other below thematerial or the belt, charges the rollers 67, 69 with oppositepolarities. An electric field (not shown) from the rollers 67, 69 theninduces a corona discharge on electrodes 71, 73 which spray the chargeonto the belts 65 which are opposite in polarity to the charge on therollers 67, 69. Use of the powered type electrostatic generator 64described above may be desirable in connection with cure tools 22 thathave complex or highly contoured tool surfaces, and with both large andsmall tool strings.

A further embodiment of apparatus for curing a composite part layup 20that reduces or eliminates part porosity is illustrated in FIG. 5. Inthis example, the bottom side 36 of the tool 22 is substantially coveredby a layer of a first material 42. A layer 56 of a second materialcovers the layer 42 of the first material and is sandwiched betweenlayer 42 and the insulating layer 28. The first and second layers of 42,56 have inherent charges that are sufficiently different on thetriboelectric scale to produce the desired static electric charge on thetool surface 25. For example, layer 42 contacting the bottom side 36 ofthe tool 22 may comprise an FEP/Teflon material having a relativelynegative charge, and layer 56 may be comprise a nylon or polyesterhaving a relatively positive charge on the triboelectric series. In theembodiment shown in FIG. 5, it is not necessary to pass air over eitherof the layers 42, 56 as is carried out in the embodiment shown in FIGS.2 and 3.

Attention is now directed to FIG. 6 which broadly illustrates theoverall steps of a method for curing a composite part layup that reducestool side porosity in cured part, and which may be carried out usingconventional vacuum bag processing techniques and a conventional curingoven. Beginning at 66, an uncured part layup 20 is placed on the surface25 of a tool 22. At 68, a vacuum bag 32 is placed over the part layup 20and sealed to the tool 22. At 70, a vacuum is drawn within the vacuumbag 32, and at 72, the tool 22 is electrically charged, which in theillustrated example comprises placing a negative charge on a tool 22,which results in a negative electrostatic charge on the tool surface 25.At 74, the electrostatic charge placed on the tool surface 25 is used toattract gas and resin molecules to the tool surface 25. At 76, thecomposite part layup is cured by placing the part layup 20 along withthe negatively charged tool 22 in a conventional oven 26.

Embodiments of the disclosure may find use in a variety of potentialapplications, particularly in the transportation industry, including forexample, aerospace, marine, automotive applications and otherapplication where automated layup equipment may be used. Thus, referringnow to FIGS. 7 and 8, embodiments of the disclosure may be used in thecontext of an aircraft manufacturing and service method 78 as shown inFIG. 7 and an aircraft 80 as shown in FIG. 8. Aircraft applications ofthe disclosed embodiments may include, for example, without limitation,curing of composite resin parts such as, without limitation beams, sparsand stringers, to name only a few. During pre-production, exemplarymethod 78 may include specification and design of the aircraft 80 andmaterial procurement 84. During production, component and subassemblymanufacturing 86 and system integration 88 of the aircraft 80 takesplace. Thereafter, the aircraft 80 may go through certification anddelivery 90 in order to be placed in service 92. While in service by acustomer, the aircraft 80 is scheduled for routine maintenance andservice 94, which may also include modification, reconfiguration,refurbishment, and so on.

Each of the processes of method 78 may be performed or carried out by asystem integrator, a third party, and/or an operator (e.g., a customer).For the purposes of this description, a system integrator may includewithout limitation any number of aircraft manufacturers and major-systemsubcontractors; a third party may include without limitation any numberof vendors, subcontractors, and suppliers; and an operator may be anairline, leasing company, military entity, service organization, and soon.

As shown in FIG. 8, the aircraft 80 produced by exemplary method 78 mayinclude an airframe 96 with a plurality of systems 98 and an interior100. Examples of high-level systems 98 include one or more of apropulsion system 102, an electrical system 104, a hydraulic system 106,and an environmental system 108. Any number of other systems may beincluded. Although an aerospace example is shown, the principles of thedisclosure may be applied to other industries, such as the marine andautomotive industries.

Systems and methods embodied herein may be employed during any one ormore of the stages of the production and service method 78. For example,components or subassemblies corresponding to production process 86 maybe fabricated or manufactured in a manner similar to components orsubassemblies produced while the aircraft 80 is in service. Also, one ormore apparatus embodiments, method embodiments, or a combination thereofmay be utilized during the production stages 86 and 88, for example, bysubstantially expediting assembly of or reducing the cost of an aircraft80. Similarly, one or more of apparatus embodiments, method embodiments,or a combination thereof may be utilized while the aircraft 80 is inservice, for example and without limitation, to maintenance and service94.

The description of the different advantageous embodiments has beenpresented for purposes of illustration and description, and is notintended to be exhaustive or limited to the embodiments in the formdisclosed. Many modifications and variations will be apparent to thoseof ordinary skill in the art. Further, different advantageousembodiments may provide different advantages as compared to otheradvantageous embodiments. The embodiment or embodiments selected arechosen and described in order to best explain the principles of theembodiments, the practical application, and to enable others of ordinaryskill in the art to understand the disclosure for various embodimentswith various modifications as are suited to the particular usecontemplated.

What is claimed is:
 1. An apparatus for curing a composite resin part,comprising: a tool having a tool surface adapted to contact a compositeresin part layup; and an electrical charge generator for generating acharge on the tool surface sufficient in magnitude to attract gasmolecules in the composite resin part layup to the tool surface.
 2. Theapparatus of claim 1, wherein the electrical charge generator includes atriboelectric charge generator.
 3. The apparatus of claim 2, wherein thetriboelectric charge generator includes at least two materials incontact with each other and sufficiently separated in the triboelectricseries to produce the electric charge on the tool surface.
 4. Theapparatus of claim 2, wherein the triboelectric charge generator islocated on the tool.
 5. The apparatus of claim 2, wherein thetriboelectric charge generator includes: a first material contacting thetool and having a inherently negative charge in a triboelectric series,and a second material contacting the first material and separated fromthe first material in the triboelectric series by an amount sufficientto generate the electric charge on the tool surface.
 6. The apparatus ofclaim 5, wherein: the first material includes first and second layers,and the second material is disposed between the first and second layersand is pervious to allow flow of air across surfaces of the first andsecond layers.
 7. The apparatus of claim 6, wherein the second materialincludes a flexible bag covering the first material and sealed to thetool.
 8. The apparatus of claim 5, wherein the first material is an FEP,and the second material is one of a nylon and a polyester.
 9. Theapparatus of claim 7, wherein: the flexible bag includes an air inlet,and the electrical charge generator includes a Venturi device fordrawing air from the air inlet across the first material and out of theflexible bag.
 10. The apparatus of claim 1, further comprising: a rackadapted to support the tool within a curing oven; and a layer ofelectrical insulation between the electric charge generator and therack.
 11. The apparatus of claim 1, wherein the electrical chargegenerator comprises a powered electrostatic charge generator.
 12. Theapparatus of claim 11, wherein the powered electrostatic chargegenerator is a Van de Graaff generator.
 13. The apparatus of claim 11,wherein the tool surface comprises at least one curve.
 14. The apparatusof claim 11, wherein the tool surface comprises at least one contour.15. The apparatus of claim 5, the tool further comprising: a bottom sideopposite the tool face; a layer of the first material substantiallycovering the tool face; a layer of the second material covering thelayer of the first material; and an insulating layer, wherein the layerof the second material is sandwiched between the layer of the firstmaterial and the insulating layer.
 16. The apparatus of claim 5, furthercomprising a vacuum bag.
 17. The apparatus of claim 16, wherein thevacuum bag is sealed to the tool surface, the vacuum bag beingconfigured to apply compaction pressure to the composite resin partduring a cure process of the composite resin part.
 18. The apparatus ofclaim 16, wherein the vacuum bag comprises a third material selectedfrom the group consisting of nylon, polyester, and combinations thereof.19. The apparatus of claim 18, wherein the third material is identicalto the second material.
 20. The apparatus of claim 18, wherein thesecond material in the third material have inherent charges on thetriboelectric scale that are substantially similar.